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June 20, 2011 -- The largest study ever to track childhood food allergies in the U.S. shows that they may be more common and more dangerous than previously recognized.

The study, a detailed survey of families with at least one child younger than 18, shows that 8% of kids under age 18 are allergic to at least one food. Surveys for about 38, 000 children were completed.

Previous studies, including a government survey published in 2009, had pegged that number at around 4%.

Allergies to peanuts were the most commonly reported, affecting 2% of kids. Milk and shellfish allergies ranked second and third. Tree nuts, egg, fin fish, strawberry, wheat, and soy rounded out the top nine food triggers.

"This study shows that there's a very high, and higher than we thought, prevalence of food allergy in the U.S." says Susan Schuval, MD, pediatric allergist at Cohen Children's Medical Center in New Hyde Park, N.Y., who was not involved in the study.

"We see this in our clinic," Schuval says, "tons and tons of food allergies."

Severe Reactions Are Common

Many food allergies in children are mild and fade over time. But in other cases, reactions to food can be dangerous and even deadly.

The new study offers one of the first estimates of these severe reactions in children, showing that 40% of kids with food allergies experience severe symptoms such as wheezing and anaphylaxis, which is characterized by difficulty breathing and a sudden drop in blood pressure.

Teenagers, however, particularly boys, were most likely to experience severe, life-threatening reactions.

"More fatalities occur in teenagers and older children," Gupta says. "They're going out with their friends and they don't want to feel different. They may not ask the ingredients in everything, you know, at a restaurant, in front of people."

Independent experts praised the scope of the new study, which is published in the journal Pediatrics.

"This is a very significant study since accurate data on the prevalence of food allergy are lacking," says Robert Wood, MD, director of pediatric allergic and immunology at Johns Hopkins University School of Medicine, in Baltimore, in an email to WebMD.

Tracking Food Allergies in Kids

Because the study was so large, researchers weren't able to use clinical measures, like blood tests or medical records to count allergy cases.

Instead, they relied on parents to report either a doctor's diagnosis or classic symptoms.

The survey, which randomly sampled parents across the country, was designed by a panel of allergy expert. The panel agreed on what symptoms to include as allergic reactions.

When symptoms reported by parents didn't match, researchers discounted the reports. For example, reports of bloating after drinking milk, which may be more indicative of lactose intolerance than a true milk allergy, were dropped.

Still, experts said, because the study didn't include objective measures, the numbers may have been skewed.

"The overall prevalence estimate of 8% seems on the high side compared to most of the prior estimates," says Scott H. Sicherer, MD, a professor of pediatrics at the Jaffe Food Allergy Institute at Mount Sinai Medical School in New York City.

Previous studies, however, were smaller and had other significant limitations. For example, one survey relied on a single question to tally allergies. Other studies only looked at allergies from a select trigger, like peanuts.

And because the current study didn't track reports of allergies over time, it's impossible to say whether the new number represents an increase in food allergies in kids.

"This is a unique study because it was large, evaluated many different foods, and gives some insights on severity and risk of food allergies in children," says Sicherer, who has reviewed other estimates of food allergies in children but was not involved in the current research.

Experts say study is also important because it hints at some of the misery that is visited on children with food allergies.

"Children who are peanut allergic are relegated to the peanut-free table at school, which kind of makes them feel like outcasts," says Schuval. "Plus there's a fear of having an allergic reaction after eating certain foods or going to a restaurant. It really can affect your whole life."